1. Field of the Invention
The present invention relates generally to the field of biomedical analysis, and particularly to an apparatus and method for non-invasively determining the cardiac output in a living subject using impedance cardiography.
2. Description of Related Technology
Noninvasive estimates of cardiac output (CO) can be obtained using impedance cardiography. Strictly speaking, impedance cardiography, also known as thoracic bioimpedance or impedance plethysmography, is used to measure the stroke volume of the heart. As shown in Eqn. (1), when the stroke volume is multiplied by heart rate, cardiac output is obtained.
CO=stroke volumexc3x97heart rate.xe2x80x83xe2x80x83(1)
The heart rate is obtained from an electrocardiogram. The basic method of correlating thoracic, or chest cavity, impedance, ZT(t), with stroke volume was developed by Kubicek, et al. at the University of Minnesota for use by NASA. See, e.g., U.S. Reissue Pat. No. 30,101 entitled xe2x80x9cImpedance plethysmographxe2x80x9d issued Sep. 25, 1979, which is incorporated herein by reference in its entirety. The method generally comprises modeling the thoracic impedance ZT(t) as a constant impedance, Zo, and time-varying impedance, xcex94Z (t), as illustrated schematically in FIG. 1. The time-varying impedance is measured by way of an impedance waveform derived from electrodes placed on various locations of the subject""s thorax; changes in the impedance over time can then be related to the change in fluidic volume (i.e., stroke volume), and ultimately cardiac output via Eqn. (1) above.
Despite their general utility, prior art impedance cardiography techniques such as those developed by Kubicek, et al. have suffered from certain disabilities. First, the distance (and orientation) between the terminals of the electrodes of the cardiography device which are placed on the skin of the subject is highly variable; this variability introduces error into the impedance measurements. Specifically, under the prior art approaches, individual electrodes 200 such as that shown in FIGS. 2a and 2b, which typically include a button xe2x80x9csnapxe2x80x9d type connector 202, compliant substrate 204, and gel electrolyte 206 are affixed to the skin of the subject at locations determined by the clinician. Since there is no direct physical coupling between the individual electrodes, their placement is somewhat arbitrary, both with respect to the subject and with respect to each other. Hence, two measurements of the same subject by the same clinician may produce different results, dependent at least in part on the clinician""s choice of placement location for the electrodes. It has further been shown that with respect to impedance cardiography measurements, certain values of electrode spacing yield better results than other values.
Additionally, as the subject moves, contorts, and/or respirates during the measurement, the relative orientation and position of the individual electrodes may vary significantly. Electrodes utilizing a weak adhesive may also be displaced laterally to a different location on the skin through subject movement, tension on the electrical leads connected to the electrodes, or even incidental contact. This so-called xe2x80x9cmotion artifactxe2x80x9d can also reflect itself as reduced accuracy of the cardiac output measurements obtained using the impedance cardiography device.
A second disability associated with prior art impedance cardiography techniques relates to the detection of a degraded electrical connection or loss of electrical continuity between the terminals of the electrode and the electrical leads used to connect thereto. Specifically, as the subject moves or sweats during the measurement, the electrolyte of the electrode may lose contact with the skin, and/or the electrical leads may become partially or completely disconnected from the terminals of the electrode. These conditions result at best in a degraded signal, and at worst in a measurement which is not representative of the actual physiological condition of the subject.
Another significant consideration in the use of electrodes as part of impedance cardiographic measurements is the downward or normal pressure applied to the subject in applying the electrode to the skin, and connecting the electrical leads to the electrode. It is desirable to minimize the amount of pressure needed to securely affix the electrode to the subject""s skin (as well as engage the electrical lead to the electrode), especially in subjects whose skin has been compromised by way of surgery or other injury, since significant pressure can result in pain, and reopening of wounds.
It is also noted that it is highly desirable to integrate cardiac output measurement capability into a compact, rugged, and efficient platform which is readily compatible with different hardware and software environments. The prior art approach of having a plurality of different, discrete stand-alone monitors which include, for example, a dedicated, redundant display and/or other output or storage device is not optimal, since there is often a need to conserve space at the subject""s bedside or even in their home (e.g., in outpatient situations), as well as cost efficiency concerns. Furthermore, a plurality of discrete stand-alone monitors necessarily consume more electrical power (often each having their own separate power supplies), and require the subject or clinician to remain proficient with a plurality of different user interface protocols for the respective monitors. In many cases, the individual stand-alone monitors are also proprietary, such that there is limited if any interface between them for sharing data. For example, where two such monitors require a common parametric measurement (e.g., ECG waveform or blood pressure), one monitor frequently cannot transmit this data to the other monitor due to the lack of interface, thereby necessitating repeating the measurement.
Recognizing these deficiencies, more recent approaches have involved the use of modular devices, wherein for example a common monitor/display function is utilized for a variety of different functional modules. These modules are generally physically mounted in a rack or other such arrangement, with the common monitor/display unit also being mounted therein. A common power supply is also generally provided, thereby eliminating the redundancy and diversity previously described. However, heretofore, impedance cardiography (ICG) equipment has not been made in such modular fashion, nor otherwise compatible with other modular devices (such as blood pressure monitoring or ECG equipment), such that such other signals can be obtained directly or indirectly from these devices and utilized within the ICG apparatus. The quality or continuity of these signals, whether obtained directly from the subject being monitored or from other modules, has not been readily and reliably provided for either.
Typical patient monitors include modules for several physiologic measurements such as ECG, blood pressure, temperature, and arterial pulse oximetry. The addition of ICG provides the physician with additional useful clinical information about the patient.
Furthermore, prior art ICG devices (modular or otherwise) do not provide the facility for direct transmission of the data obtained from the subject, or other parameters generated by the ICG device after processing the input data, to a remote location for analysis or storage. Rather, the prior art approaches are localized to the bedside or monitoring location. This is a distinct disability with respect to the aforementioned outpatient applications, since the subject being monitored must either manually relay the information to the caregiver (such as by telephone, mail, or visit), or perform the analysis or interpretation themselves. Additionally, it is often desirable to perform more sophisticated (e.g. algorithmic) comparative or trend analysis of the subject""s data, either with respect to prior data for that same subject, or data for other subjects. The lack of effective transmission modes in the prior art to some degree frustrates such analysis, since even if the subject has the facility to perform the analysis (e.g., PC or personal electronic device with the appropriate software), they will not necessarily be in possession of their own prior data, which may have accumulated via monitoring at a remote health care facility, or that for other similarly situated subjects.
Based on the foregoing, there is a need for an improved apparatus and method for measuring cardiac output in a living subject. Such improved apparatus and method ideally would allow the clinician to repeatedly and consistently place the electrodes at the optimal locations. Additionally, such an improved apparatus and method would also permit the detection of degraded electrical continuity between the electrode terminal and skin, or the electrode terminal and electrical leads of the measurement system, and be adapted to minimize the normal pressure on the subject""s tissue when applying the electrodes and electrical leads. The apparatus would further be adapted to interface with other monitoring/display systems and parametric measurement modules that may be coincidently in use, and have connectivity beyond the immediate locale of the apparatus to permit the ready transfer of data to and from one or more remote locations or devices. Facility for selecting the highest quality input from a number of different sources would also ideally be provided.
The present invention satisfies the aforementioned needs by providing an improved method and apparatus for measuring the cardiac output of a living subject.
In a first aspect of the invention, an improved apparatus for measuring the cardiac output (CO) of a living subject is disclosed. In one exemplary embodiment, the living subject is a human being, and the apparatus comprises a system having a plurality of electrode pairs, a constant current source, a plurality of electrical leads connecting the constant current source with the plurality of electrode pairs, a differential amplifier for measuring the differential voltage at the electrodes, and circuitry for measuring ECG potentials from the electrode pairs. A predetermined distance is maintained between each of the individual electrodes in each electrode pair, thereby mitigating error sources relating to the relative placement of individual electrodes from the cardiac output measurement. Cardiac stroke volume is measured using the aforementioned apparatus by applying a constant current to the stimulation electrodes, measuring the resulting voltage differential at the measurement electrodes, and determining the stroke volume from the measured voltage and a predetermined relationship describing intra-thoracic impedance. The system also measures cardiac rate via the ECG potentials at the electrodes; cardiac output is then determined using the measured cardiac stroke volume and cardiac rate from the ECG potential.
In a second aspect of the invention, an improved cardiac output electrode assembly is disclosed. In one exemplary embodiment, the electrode assembly comprises a pair of electrode terminals disposed a predetermined distance from one another within an insulating substrate using a xe2x80x9csnapxe2x80x9d arrangement and electrolytic gel interposed between the electrode and skin of the subject. The substrate and gel materials of the electrode assembly are advantageously selected so as to provide a uniform and firm physical contact of the gel (and accordingly the electrode terminals) with the skin of the patient, and position of the terminals with relation to one another. The predetermined spacing of the electrodes also facilitates the detection of discontinuities in the system (such as an electrode becoming disconnected from the patient) through the measurement and comparison of impedance waveforms obtained from various electrode terminals. Additionally, the electrode pairs are used in conjunction with connectors having opposable jaws adapted such that no downward or normal pressure need be applied when a connector is fastened to an electrode terminal, yet consistent electrical properties are maintained.
In a third aspect of the invention, an improved method of measuring the cardiac output of a living subject is disclosed. The method generally comprises providing a plurality of electrode pairs; positioning the electrode pairs at predetermined locations above and below the thoracic cavity, generating a constant current; applying the constant current to one electrode of each of the electrode pairs; measuring the voltage at the second electrode of each electrode pair; determining cardiac stroke volume from the measured voltage; and determining cardiac output based on stroke volume and cardiac rate. In one exemplary embodiment, four electrode pairs are utilized, each having a predetermined spacing between both of the individual electrodes of the pair. The electrode pairs are placed at locations above and below the thoracic cavity of the subject, on both the right and left sides. Both differential voltage (related to the time-variant component of total thoracic impedance) and cardiac rate are measured via the electrode pairs for both sides of the subject.
In a fourth aspect of the invention, an improved method of monitoring the electrical continuity of a plurality of electrodes in an impedance cardiography system is disclosed. In one exemplary embodiment, the method comprises providing a plurality of electrically conductive terminals; disposing the terminals in relation to the thoracic cavity of a subject; generating a current between a first terminal and a second terminal, the current passing through at least a portion of the thoracic cavity; measuring an impedance waveform from the second terminal; and comparing the measured impedance waveform to a similar waveform measured from another terminal, the difference between the impedance waveforms being used to evaluate the electrical continuity of the first terminal.
In a fifth aspect of the invention, ant improved impedance cardiography (ICG) module adapted to implement various of the foregoing aspects is disclosed. The module comprises a plurality of interfaces adapted to receive signals such as impedance and ECG waveforms; determinations of cardiac output (CO) and other related parameters are output via another interface to a host or monitoring device. In one exemplary embodiment, the module of the present invention comprises a digital processor-based device adapted to process impedance and other signals derived from one or more living subjects, and output signals to the monitor/display unit according to an established communications protocol. A microprocessor/DSP architecture is used in conjunction with signal filtration, analog-to-digital conversion, and other signal conditioning/processing within the module to extract useful cardiographic information from the patient signals received via the interfaces, and communicate this information to the monitor/display unit or other output device under control of the microprocessor. Other inputs such as the subject""s blood pressure, multiple ECG waveforms, and the like may be utilized by the module during the aforementioned CO determination. The module may further be configured to generate the stimulation signal (e.g., constant current previously described) which is provided to the patient electrodes. In another embodiment, the module is further configured for impedance cardiographic (ICG) and electrocardiographic (ECG) waveform fiducial point detection and analysis using discrete wavelet transforms.
In yet another embodiment, the module includes a network interface adapted to couple the module to a data network capable of distributing the data generated by the module and other devices to local and/or remote network nodes, such as local stations within a health care facility, or to a remote health care or medical facility in the case of outpatient applications. Communication with other network nodes, locations, or personal electronic devices is accomplished using, for example, modulator/demodulator (modem) apparatus, wireless interface such as Bluetooth(trademark), local- or wide-area network (LAN/WAN) topologies, circuit or packet-switched high-bandwidth data networks (such as asynchronous transfer mode), internet, intranet, the Wireless Medical Telemetry Service (WMTS) medical band (608-614 MHz), synchronous optical networks (SONET), FDDI, or even satellite communications.
In yet another embodiment, the ICG module of the invention comprises a yoke adapted to interface with a fixed or mobile electrocardiograph system. In one exemplary embodiment, the yoke is highly mobile and is adapted to electrically interface with the leads attached to the subject, as well as with the host monitor. The yoke further may include indications of the operating status of the yoke, as well as other data interfaces for transmitting ICG data to, and receiving other types of data (such as blood pressure data) from, other processing modules.
In yet another embodiment, the ICG module apparatus is configured to operate in conjunction with a dialysis (e.g., hemodialysis) system. Because cardiovascular events account for over 50% of deaths in dialysis patients per year in the United States, more vigilant cardiovascular disease management, including hemodynamic monitoring through impedance cardiography, may increase the survival rate of this patient population. The ICG module is adapted to receive data such as patient blood pressure from the dialyzer or one of the dialyzer""s modules, which may operate contemporaneously with the ICG module while the patient is being dialyzed and monitored.
In yet another embodiment, the improved ICG module of the invention comprises a card or board level plug-in module adapted for receipt within a host device such as a personal computer or dedicated monitor/display unit.
In a sixth aspect of the invention, an improved method of waveform selection for input to the module processing is disclosed. In one exemplary embodiment, the input waveforms comprise ECG waveforms used in the CO determination, and the method comprises evaluating each waveform for signal quality based upon at least one parameter; ranking each waveform based on the foregoing quality evaluation; and selecting the one waveform with the best rank for further processing. The Q and R fiducial points are used to determine the quality evaluation parameters, which may include for example R-wave amplitude, QR interval difference, and RR interval difference. In this fashion, the module of the present invention evaluates the various sources of input data, and selects the best one from the available signals for further signal processing.
In a seventh aspect of the invention, an improved software environment adapted for use with the aforementioned ICG module is disclosed. In one exemplary embodiment, the software environment comprises initialization, operating, and processing modules adapted to perform various start-up, signal processing, communication, and error detection functions within the module.